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question:10 cdot 21 The sum of the first 10 terms and the sum of the first 100 terms of an arithmetic sequence are 100 and 10, respectively. What is the sum of the first 110 terms?(A) 90.(B) -90.(C) 110.(D) -110.(E) -100.(31st American High School Mathematics Examination, 1980)

answer:[Solution] The formula for the sum of the first n terms of an arithmetic sequence S_{n} is2 S_{n}=n[2 a+(n-1) d], where the first term is a, and the common difference is d.From the problem, we havebegin{array}{c}200=10(2 a+9 d) text { and } quad 20=100(2 a+99 d), 2 S_{110}=110(2 a+109 d) .end{array}From the first two equations, we get quad 2 a+109 d=-2.Therefore, 2 S_{110}=110 cdot(-2), so S_{110}=-110. Hence, the answer is (D).

question:Rectangle ABCD has side lengths AB=84 and AD=42. Point M is the midpoint of overline{AD}, point N is the trisection point of overline{AB} closer to A, and point O is the intersection of overline{CM} and overline{DN}. Point P lies on the quadrilateral BCON, and overline{BP} bisects the area of BCON. Find the area of triangle CDP.

answer:Impose a coordinate system on the diagram where point D is the origin. Therefore A=(0,42), B=(84,42), C=(84,0), and D=(0,0). Because M is a midpoint and N is a trisection point, M=(0,21) and N=(28,42). The equation for line DN is y=frac{3}{2}x and the equation for line CM is frac{1}{84}x+frac{1}{21}y=1, so their intersection, point O, is (12,18). Using the shoelace formula on quadrilateral BCON, or drawing diagonal overline{BO} and using frac12bh, we find that its area is 2184. Therefore the area of triangle BCP is frac{2184}{2} = 1092. Using A = frac 12 bh, we get 2184 = 42h. Simplifying, we get h = 52. This means that the x-coordinate of P = 84 - 52 = 32. Since P lies on frac{1}{84}x+frac{1}{21}y=1, you can solve and get that the y-coordinate of P is 13. Therefore the area of CDP is frac{1}{2} cdot 13 cdot 84=boxed{546}.

question:Evaluate the integral int_{0}^{4}|x-2|dx = ______.

answer:First, we need to split the integral into two parts at x=2, where the absolute value function changes its behavior. Therefore, we have:int_{0}^{4}|x-2|dx = int_{0}^{2}(2-x)dx + int_{2}^{4}(x-2)dx.Now, let's evaluate each integral separately.For the first integral, we have:int_{0}^{2}(2-x)dx = left[2x - frac{1}{2}x^2right]_0^2 = (4 - 2) - (0 - 0) = 2.For the second integral, we have:int_{2}^{4}(x-2)dx = left[frac{1}{2}x^2 - 2xright]_2^4 = (8 - 8) - (2 - 4) = 2.Finally, adding the two results together, we obtain:int_{0}^{4}|x-2|dx = 2 + 2 = boxed{4}.

question:Simplify first, then evaluate: [left(3x+2yright)left(3x-2yright)-left(3x-2yright)^{2}]div left(4yright). Given x=frac{1}{2} and y=-frac{1}{4}.

answer:To simplify and then evaluate the given expression [left(3x+2yright)left(3x-2yright)-left(3x-2yright)^{2}]div left(4yright) with x=frac{1}{2} and y=-frac{1}{4}, we follow these steps:1. Simplify the expression inside the brackets: begin{align*} [left(3x+2yright)left(3x-2yright)-left(3x-2yright)^{2}] &= [9x^{2}-4y^{2}-(9x^{2}-12xy+4y^{2})] &= 9x^{2}-4y^{2}-9x^{2}+12xy-4y^{2} &= 12xy-8y^{2} end{align*}2. Divide the simplified expression by (4y): begin{align*} (12xy-8y^{2})div left(4yright) &= frac{12xy}{4y} - frac{8y^{2}}{4y} &= 3x - 2y end{align*}3. Substitute (x=frac{1}{2}) and (y=-frac{1}{4}) into the simplified expression: begin{align*} 3x - 2y &= 3left(frac{1}{2}right) - 2left(-frac{1}{4}right) &= frac{3}{2} + frac{1}{2} &= 2 end{align*}Therefore, after simplifying and substituting the given values of (x) and (y), the final answer is boxed{2}.

question:Given xy=-2, x+y=4, find the value of the algebraic expression x^{2}y+xy^{2}.

answer:Given the equations xy=-2 and x+y=4, we aim to find the value of the expression x^{2}y+xy^{2}.Starting with the given expression:[x^{2}y+xy^{2}]We can factor out the common term xy:[= xy(x+y)]Substituting the given values for xy and x+y:[= (-2) times 4]Performing the multiplication:[= -8]Therefore, the value of the algebraic expression x^{2}y+xy^{2} is boxed{-8}.

question:For what real number m is the complex number z=m^2+m-2+(m^2-1)i (1) a real number; (2) an imaginary number; (3) a pure imaginary number?

answer:(1) If the complex number z is a real number, then m^2-1=0, so m=pm1; (2) If the complex number z is an imaginary number, then m^2-1neq 0, so mneqpm1; (3) If the complex number z is a pure imaginary number, then begin{cases} m^2+m-2=0 m^2-1neq 0 end{cases}, so m=-2.Therefore, the answers are:(1) m=pm1 for z to be a real number, so boxed{m=pm1}; (2) mneqpm1 for z to be an imaginary number, so boxed{mneqpm1}; (3) m=-2 for z to be a pure imaginary number, so boxed{m=-2}.

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